Circuits have been developed in the past to provide automatic switching of a primary power supply voltage to a battery powered backup power supply when the primary power source has failed. Generally, the batteries used with these circuits have a limited useful life which is much less than the useful life of the circuit that they are attached to. Therefore, these batteries require periodic checking and maintenance.
Recently, however, long-life lithium energy cells have been developed which have a shelf life of many years. These lithium energy cells are relatively small compared to older battery types; and it is possible to combine a lithium energy cell with a microcomputer and a static random access memory into a dual in-line integrated circuit package. In these circuits, the backup battery provides backup power to both the microcomputer and the static RAM and is able to do so for many years.
However, since the lifetime of these lithium energy cells is limited, it is desirable to reduce the drain on the lithium batteries by removing the battery voltage from the static RAM at times when it is not necessary to preserve the data stored in the static RAM such as, for example, when the part is being stored in inventory prior to the time when it is programmed and actually used.
Therefore, it can be appreciated that a backup battery switching circuit for use in a microcomputer or microprocessor which allows the backup battery to be disconnected from a static RAM when it is desired to restrict the drain on the backup battery is highly desirable.